1. Field of the Invention
This invention relates to an electroviscous fluid which is increased in viscosity by application of a voltage.
2. Background Information
An electroviscous fluid is a suspension in which finely divided hydrophilic solids are dispersed in a hydrophobic and non-electroconductive oil, which is increased in viscosity of the fluid extremely rapidly and yet reversibly under the action of a sufficiently strong electric field to become a plastic or solid state.
For varying the viscosity, not only a direct current electric field, but also an alternate current electric field can be used, and a strong force can be given with a very small necessary current, little power, and therefore electroviscous fluids have been used as the constituent element in, for example, the clutch, hydraulic valve, shock absorber, vibrator, antivibration rubber or the electrical-mechanical interface for controlling the system to hold a workpiece at a normal position, etc.
In the prior art, as the dispersed phase constituents of an electroviscous fluid, solid fine particles have been employed, and as the solid fine particles, cellulose, starch, silica gel, ion-exchange resin, etc. with water adsorbed on the surface and formed into fine particles have been known. As the liquid phase component, which is the other component, PCB, butyl sebacate, transformer oil, chlorinated paraffin, silicone oil, etc. have been known. However, these materials are poor in practical value, and there exists yet no electroviscous fluid of extremely high performance and high stability which is of available practical value.
One reason why no electroviscous fluid has been provided for practical application is because the specific gravity of the solid fine particles, which become the dispersed phase, is generally greater than that of the liquid phase component, whereby phase separation occurs when left to stand for a long term to effect sedimentaion, thereby forming precipitates which can be dispersed again with difficulty.
As a means for solving such problem, there is the method in which solid fine particles with a small specific gravity is employed to make the difference in specific gravity from the liquid phase component smaller and the method in which a liquid phase component with larger specific gravity is employed to make the difference in specific gravity from the solid fine particles smaller.
However, in the case of the former method, solid fine particles are limited to an organic material, such as starch, and therefore electrical characteristics are lacking in stability over a long term and practical applicability becomes poor.
Accordingly, when an improvement of electroviscous effect is aimed at, as disclosed in U.S. Pat. No. 4,129,513, solid fine particles with large specific gravity to some extent cannot but be used. However, lithium polyacrylate used as the dispersed phase fine particles in the above published specification has a specific gravity of about 1.4 to 1.5, and therefore no hydrocarbon type oil with a small specific gravity generally employed as an electrically insulating oil cannot be used.
Accordingly, when solid fine particles with a somewhat great specific gravity are to be used for the purpose of improving the electroviscous effect, the latter method has been employed. As an example of the latter method, there is the method disclosed in U.S. Pat. No. 4,502,973. However, in the above published specification, a compound with a great specific gravity, such as a halogenated diphenylmethane similar in structure to PCB, the use of which is prohibited as being very harmful to the human body, is employed as the liquid phase, and therefore this technique is not satisfactory in practical applicability.
Also, in many constituent elements utilizing an an electroviscous fluid, electroviscous fluid is used in direct contact with a material having a rubbery elasticity, and therefore one may consider to employ an electrically insulating oil, such as silicone oil, which does not swell or a denatured rubbery elastic material as the liquid phase. In DE-3427499, from this standpoint, an electroviscous fluid is proposed in which silica gel is used as the dispersed phase and silicone oil as the liquid phase, and further an amino functional, hydroxy functional, acetoxy functional or alkoxy functional polysiloxane group is used as the dispersing agent. However, due to the great difference in specific gravity beween silia gel and silicone oil, the particles of the dispersed phase are sedimented when left to stand for a long term, to cause phase separation and can be dispersed again with difficulty.
Also, in Japanese Patent Publication No. 31221/1985, there is disclosed the method in which an ion-exchange resin formed into fine powder is used as the dispersed phase. However, this technique uses a resin comprising a crosslinked styrene polymer as the basic skelton, which has no affinity for silicone oil, and when phase separation is once generated, redispersion becomes difficult. Thus, these fluids are also practically unavailable under the present situation.
Accordingly, an object of the present invention is to develop an electroviscous fluid which solves the above problems, is stable for a long term and can exhibit a high electroviscous effect.